An optical viewing system and a method for operating an optical viewing system of the kind mentioned above are known from United States patent application publication US 2001/0024319 A1. Here, a surgical microscope is described which has a unit for reflecting in data. This unit for reflecting data in includes a display with which image data are generated. The image data are superposed on the image of a viewing region via a beam splitter. The superposed image is then supplied to an image sensor for documentation and illustration purposes. To prevent a flickering of the reflected-in data, it is described in United States patent application publication US 2001/0024319 A1to synchronize the image sensor and the corresponding display.
German patent publication 100 20 279 discloses a stereo microscope having an ocular unit which makes possible the stereoscopic investigation of an object region for a user with a binocular viewing beam path. The ocular unit is combined with a display unit and an image recording unit which make it possible to supply an image, which is reflected into a viewing beam path by means of a beam splitter, to the eye of the viewer as a superposition onto an image of the object region as well as to record this superposed image with an image recording unit.
A surgical microscope having a unit for reflecting in data and having a video camera is described in German utility model registration 299 23 951. This surgical microscope has a first beam splitter cube with which a display image is reflected into a viewing beam path in order to be displayed in a left primary beam path. Furthermore, the surgical microscope has a second beam splitter cube via which the video camera is connected to the viewing beam path. This second beam splitter cube makes possible that the video camera can record object images via a right primary viewing beam path.
As displays for showing image data, so-called modular displays are known which are illuminated with light from one light source or from several separate light sources and which modulate this light pixel for pixel. An example of such displays are so-called LCOS displays. One such LCOS display includes, for example, a nematic fluid crystal which is mounted on a silicon substrate (LCOS=liquid crystal on silicon). Electric conductor paths and component groups are disposed on the silicon substrate. For a corresponding drive, the conductor paths and component groups make it possible to locally adjust the polarization characteristics of the liquid crystal for light and to there quasi continuously tune the polarization characteristics. A polarization beam splitter is usually assigned to such an LCOS display through which the display is illuminated. The polarization beam splitter enables the light reflected by the LCOS display. Alternatively, it is also possible to illuminate the LCOS display via a first pole filter and to supply the light, which is reflected by the LCOS display, to a viewer via a second pole filter. When such an LCOS display is illuminated then the intensity of the light, which is reflected by the LCOS display, can be practically continuously tuned between darkness and maximum reflection via corresponding changes of the polarization characteristics of the nematic liquid crystal.
Furthermore, so-called FLC microdisplays are known as modulating displays which contain a ferro-electric liquid crystal (FLC). This liquid crystal is arranged on a logic circuit on a silicon base which makes it possible to switch back and forth the polarization characteristics of the liquid crystal locally between two binary states in correspondence to the position of individual pixels. When such an FLC microdisplay is illuminated by a polarization beam splitter with polarized light, then the pixels of the FLC microdisplay appear either dark or bright. In order to bring about a changeable luminance impression of a pixel on an FLC microdisplay for a viewer, the polarization state of a pixel is adjusted in a pulsed manner. fThe corresponding pixel brightness results then from an integral actual luminance duration of a pixel in a characteristic time interval. Here, it is utilized that the FLC microdisplay can be driven very rapidly because it is possible to change the polarization state of a pixel on a time scale below 50 μs.
A further example for modulated displays are so-called digital mirror displays (DMD). These displays have a carrier unit on which thousands of small micromirrors are arranged. These micromirrors can be driven individually in order to change their position or orientation. One or several light sources are assigned to such a display. The light of the light sources is reflected by the micromirrors in order to generate an image on a projection surface. The micromirrors correspond to the pixels of an image generated by means of the display. Similar as in an FLC microdisplay, the brightness impression for an image pixel is brought about for a viewer in that the corresponding micromirror is pulse driven in such a manner that it generates light pulses on a suitable projection surface. The actual luminance duration of a pixel over a characteristic time interval is perceived by a viewer as a luminance impression having a defined brightness.
To display colors, it is known to generate, with a corresponding display pixel, the following: a pulse sequence train for a first complementary color; a pulse sequence train for a second complementary color; and, thereafter, a pulse sequence train for a third complementary color.
Furthermore, transmissive displays of the type “color sequential” are known which are combined with two pole filters and are operated in a transmitted-light mode.
Furthermore, so-called emissive displays are utilized as displays for showing image data. These emissive displays generate light pixel for pixel and are usually based on the principal of the vacuum fluorescence or of the field emission. In lieu of the color-sequential display, the color information is here realized via regular spatial arrangement of red, green and blue color filters in front of the individual pixels. The structure boundary of the pixels lies below the resolution limit of the eye. With a corresponding drive of the individual differently colored pixels, it is possible to generate any desired color distribution in an image. Such displays can also be based on organic luminescent diodes, so-called organic light emitting diodes (OLEDs). In such emissive displays, as a rule, similar as in LCoS displays, the intensity of the light, which is outputted by a display pixel, can be quasi-continuously tuned between darkness and a maximum value. OLED microdisplays having SVGA resolution are, for example, offered by the Emagin Company. In this display, each pixel comprises three subpixels having the colors red, green and blue. What is problematic is the comparatively low luminescent density of such displays for the use in a surgical microscope. Monochrome OLED microdisplays having a luminescent density, which is, sufficient for a surgical microscope, are, however, commercially available.
As displays, there are also so-called transmissive LCDs with color filters red-green-blue ahead of each pixel. The operation of these displays corresponds to that of a TFT-LCD monitor for computers.
When these displays are built small (that is, they have an image screen diagonal of less than 25 mm, often also less than 10 mm) and make possible a display with a high information density (for example, QVGA resolution, SVGA resolution, SXGA resolution or even higher resolution), these displays are also characterized as microdisplays.
In surgical microscopes, it is required that the area of surgery be illuminated as brightly as possible in order to obtain a surgical microscope image having good contrast. If a superposed image of the surgical area and a reflected-in display is to be recorded with a camera, for example, for documentation purposes in the surgical microscope, then this camera should be operated in a comparatively short time span for light sensitivity because of the high total image brightness. This comparatively short time span for light sensitivity has the consequence that, in a display wherein the brightness impression of individual pixels over an averaged luminance duration is caused in a characteristic time interval, the camera does not perceive the image, which is shown with the display, or only partially perceives the same because the camera is not sensitive at the time or in the time interval or in the time intervals in which the affected pixels luminesce.
This problem is also present in displays wherein the colored brightness impression of a pixel is caused by the intensity of the light emitted by a pixel insofar as the display is operated in a video mode, in that a display pixel makes available sequential luminescent pulses of a characteristic duration having different colors.
If the time for sensitivity of the camera is not coincident with the time for the luminescent pulses of the display pixels, then the component of the display image in the total image is not at all or only very poorly detected with the camera and therefore chromatic aberrations can occur.